[Developmental effects of perinatal hypothyroidism on cognitive functions: focused on attention]

Thyroid hormones are essential for the development of brain. Perinatal hypothyroidism induced by environmental endocrine disrupters is reported to cause cognitive dysfunctions such as attention and memory deficits. The purpose of this study is whether perinatal hypothyroidism causes deficits of shift attention and divided attention in offspring using a target detection task.

Pregnant rats were treated with the anti-thyroid drug methimazole [0.02% (w/v)] from gestational day 15 to postnatal day 21. The offspring received behavioral testing using shift attention and divided attention tasks. The testing of shift attention started with the presentation of one of two targets. A lever was inserted to the same side as the presented target (ipsilateral trial). However, a lever was inserted to the opposite side to the presented target with the probability of 20% (contralateral trial). Next, the testing of divided attention was introduced. The testing started with the presentation of one of the two targets (one-side trial). A lever was inserted to the same side as the presented target. However, both targets were presented (both-side trial) and one of the two levers was inserted with the probability of 20%.

On the contralateral trials in the shift attention task, the untreated rats responded frequently with reaction times of 1.4-1.6 s, whereas the treated rats responded sporadically with a wide range of reaction times of 1.2-7.2 s. This indicates that the treated rats were not able to shift their attention quickly toward the opposite side. When the both-side targets were presented in the divided attention task, the untreated rats responded frequently with the reaction times of 0.6-0.8 s and 1.4-1.6 s. The treated rats responded frequently with reaction times of 0.8-1.0 s and 1.0-6.4 s. Both the untreated and treated rats did not divide their attention toward the both-side targets but probably paid attention to one of the two targets. They responded with the shorter reaction times when the lever was inserted in the same side that they paid attention and responded with longer reaction times when the lever was inserted in the opposite side. We conclude that perinatal hypothyroidism affects shift attention in rats but further researches are necessary to examine whether hypothyroidism affects divided attention.

Perinatal hypothyroidism causes the deficit of shift attention in rats but it was unclear whether hypothyroidism affects divided attention.